In the field of scanning systems, especially in systems having components of low precision for cost competitive reasons, errors exist due to variations in the performance of the components. For example, a laser in a scanning system may have variations in output due to aging of the components or due to frequency characteristics of the power supply. Or, for example, the reflectivity of a polygon mirror may vary from facet to facet.
There is a need to correct for the variations in scanning systems such that they are capable of producing a reduced error or error-free image. Other approaches exist that correct for image errors using sensors adapted to monitor generally a sum sampling of the exposure errors. Such prior attempts have neglected to monitor the variations of the individual components separately in an appropriate manner and then compensate for the error introduced by each component by combining the individual variations or errors and adjusting for the combined errors.
For example, U.S. Pat. No. 4,400,740 (Traino et al.) describes a scanning system adapted to monitor the intensity of an exposure beam and to correct for the variation in intensity of the exposure beam. The method taught by this patent, however, monitors the beam intensity using a single sensor adapted to intercept the scanning beam for only a moment during the relatively longer period of time that the beam exposes the photoreceptor in the image plane. While such a method for controlling exposure variations might be adequate for correcting for polygon facet reflectivity error or low frequency variations in the intensity of the laser beam, the method is not adaptable to compensate for high frequency variations in laser beam intensity.
Another example is U.S. Pat. No. 4,831,247 (Ishizaka). One method taught by this patent utilizes a single detector, as described in the above referenced Traino et al. patent, but uses different control circuitry to modulate the laser beam as a function of imaging beam intensity variations caused by polygon facet reflectivity errors. As described above such an approach is not adaptable to allow a continuous monitoring of the laser beam intensity and therefore is not adaptable to compensate for high frequency variations in intensity. This patent also teaches a method for monitoring variations in polygon facet reflectivity on a continuous basis. However, the method is not concurrently capable of monitoring the imaging beam variation on a continuous basis. In addition, the method taught by Ishizaka does not separate the variations in intensity of the reference laser beam from the variations in facet reflectivity. Such a correction method has the potential of doubling the error of the exposure should the laser intensity variations of the reference laser be out of phase with the intensity variations of the imaging laser.
Montagu, "Laser Beam Scanning", Gerald F. Marshall Editor, published by Marcel Dekker, Inc. (1985), at pages 255-274, describes electronic means to correct for flat field scanning positional errors when using galvanometer and resonant mirror scanning systems. Other sources of errors, such as laser intensity stability errors etc., are mentioned. However, the article does not teach electronic means for sensing the errors individually and then combining error compensation signals in order to correct for several of the scanning errors in the system as a group.
In view of the foregoing it is believed advantageous to provide a system that monitors both continuous and incremental errors, combines the individually monitored continuous and incremental errors into a continuous system error correction signal, and applies the combined system error correction signal in order to compensate for the errors in the system.